"We have never speculated about the timing of anything during these negotiations, and we're certainly not going to start now -- especially given the fact that major issues remain to be resolved in these talks," the senior State Department official said.
Iranian diplomat Alireza Miryousefi, writing on Twitter, quoted a senior official from Tehran as saying a deal by tonight was "logistically impossible" as the agreement being drawn up spanned 100 pages.
The talks are aimed at finalising a deal to make it extremely difficult for Iran to develop nuclear weapons.
Tehran denies its nuclear programme has military aims.